This invention relates to atomic absorption spectrophotometry and more particularly to an atomic absorption spectrometer and method for correcting for variations in the light source-detector assembly.
An atomic absorption spectrometer generally comprises a light source emitting a line spectrum with the resonant lines of a looked-for element and an optical system generating a measuring light beam originating from the light source and impinging upon a detector. An atomizing-means transforms the sample substance into an atomic state so that a "cloud of atoms" is generated in the path of the measuring light beam. The atomizer means may be a burner with the liquid sample being sprayed into the burner flame. The atoms of the looked-for elements absorb the measuring light beam whereas the measuring light beam is substantially unaffected by atoms of other elements. Therefore, the measuring light beam undergoes an attenuation which depends on the quantity of the looked-for element in the cloud of atoms and thus in the sample. Atomic absorption spectroscopy is a very sensitive and very accurate method for determination of concentrations of a looked-for element in a metered sample.
Essential for this measurement, however, is an exact knowledge of the zero line, i.e., the intensity of the measuring light beam measured in the absence of the looked-for element in the sample. This zero line can change by variation of the lamp intensity or the detector sensitivity and such variations would immediately affect the measurement.
Double beam spectrophotometers are known in which a light beam originating from a light source is alternatingly directed through a measuring path and a reference path by a chopper arrangement. The sample to be measured is arranged in the measuring path and a reference sample is arranged in the reference path. The beams subsequently are recombined by a semitransparent mirror or another chopper and impinge upon a common detector. The signal obtained with the reference path then forms the zero line to which the signal obtained with the measuring path can be referenced. The change between the measuring and reference path conventionally is effected with line frequency. Such double beam spectrophotometers suffer from the disadvantage that the measuring light beam impinges upon the detector only during, at most, half of the time. When the sample is a cloud of atoms in a flame, a relatively high noise level results. Therefore, efforts have to be made to utilize the energy of the desired signal as completely as possible in order to obtain a favorable signal-to-noise ratio. Accordingly, in a double beam apparatus, a periodic fast change between a measuring path of rays and a reference path would be disadvantageous and would reduce the sensitivity of the atomic absorption spectrometer.
In EP-A-84 391, an atomic absorption spectrometer is disclosed in which a reference path of rays is provided which avoids the flame. This reference path of rays provides at the detector a signal changing only with the apparatus drift caused by variation of the lamp intensity or by variation of the detector sensitivity. This signal serves for compensation of the drift in signals obtained by the measuring path of rays through the flame. For this purpose, movable mirrors are moved into the measuring path of rays between the sample measurements. The light is directed along the reference path and reflected by mirrors from the reference path again, to the entrance slit of a common monochromator.
During the measurement, the light beam passes from the light source only through the measuring path such that the total energy of the light beam is available for the measurement and a favorable signal-to-noise ratio results. The measurement through the reference path is made while the flame stabilizes after a sample change.
The pivoting of mirrors into the path of rays of the measuring light beam in order to direct the light beam into the reference path and subsequently reflecting it back into the measuring path requires high precision of the mirrors to be pivoted. Errors in the alignment of these mirrors enter into the direction of the reflected light beam with twice the angle. This, in turn, can affect the intensity of the light beam passing through the entrance slit of the monochromator.
Accordingly, it is an object of the present invention to provide a new and improved atomic absorption spectrometer.
Another object is to provide such a spectrometer which attains an enhanced signal-to-noise ratio while compensating for light source and detector variations.
Another object of the invention is to provide such a spectrometer which avoids movable mirrors and the associated precision alignment thereof.
A further object of the invention is to provide such a spectrometer having fixed stationary optical elements defining a single fixed optical path for both sample and reference measurements.
A still further object of the invention is to provide a spectrometer having a selectively movable atomizer for selective positioning of the atomized sample within and without a fixed optical path.
Another object of the invention is to provide a new and improved method of compensating for light source-detector variations in an atomic absorption spectrometer.
Accordingly, it has been found that the foregoing and related objects and advantageous are attained in an atomic absorption spectrometer having a housing, a line emitting light source, a detector for measuring the absorption of light from the light source, and optical means for generating a measuring beam along a fixed optical path from the light source to the detector with the optical path extending through the housing. An atomizer for atomizing a sample to generate a cloud of atoms for atomic absorption analysis with the light beam is movably mounted within the housing for movement between a first sample measurement position wherein the cloud of atoms from the atomizer is within the optical path and a second reference measurement position wherein the cloud of atoms is without the optical path. A microprocessor control controls a sample measurement when the atomizer is in the first position and a reference measurement when the atomizer is in the second position to compensate the sample measurement based upon the reference measurement. The atomizer is a flame burner atomizer mounted to a moldable support carriage for movement between the sample measurement position and the reference measurement position. An adjustable mounting assembly permits selective adjustment of the mounting of the burner with respect to level and angle.
In this arrangement, the measuring light beam remains the same both during the sample measurement as during the drift compensation. The measuring light beam selectively passes uninterruptedly through the "cloud of atoms" during the measuring time and a drift compensation is effected outside the measuring time. The measuring light beam is exclusively directed by stationary, optical elements of the imaging optical system which have been accurately adjusted once and forever, and movable mirrors and the associated precision requirements thereof are avoided. Instead, the atomizer burner is displaced into and out of the stationary measuring light beam. The displacement of the burner can be effectively and reliably accomplished with relatively low costs. The guiding of the burner does not require extreme precision since the position of the burner does not have an influence on the position of the measuring light beam and the burner can be reproducibly moved into the sample measurement position with simple means.
The method of light source-detector variation compensation of the present invention includes directing a light beam for atomic absorption spectrometry along a predetermined fixed optical path between a light emitting source and a detector assembly for performing sample and reference measurements. An atomizer is positioned in a first predetermined position so that the atomized sample therefrom is in the optical path of the light beam. A sample measurement is performed with the total available energy of the light beam with the atomizer in the first position. The atomizer is positioned in a second predetermined position so that the atomized sample is without the optical path and a reference measurement is performed. The sample measurement is corrected to compensate for light source variations based upon the reference measurement.